1. Field of the Invention
The present invention relates to an operational amplifier and its offset cancel circuit, in particular, suitable for use in a source driver IC for a liquid crystal display (LCD) having a plurality of operational amplifier outputs in the same chip.
2. Description of the Related Art
In a liquid crystal panel of a liquid crystal display (LCD), one pixel is composed of three colors of red, blue, and green. Each color has, for example, 64 gradations. In this case, 64xc3x9764xc3x9764≈260,000 colors can be displayed per pixel. An LCD source driver generates gradation voltages at 64 levels for each of the three colors. Each gradation voltage is generated as positive or negative type with respect to a common electrode.
FIG. 1 is a circuit diagram showing the configuration of a conventional operational amplifier. The operational amplifier shown in FIG. 1 comprises a current mirror section comprising a pair of p-channel transistors M1 and M2; a differential input section connected to the current mirror section and comprising a pair of n-channel transistors M3 and M4; two n-channel transistors M9 and M10 which function as constant current circuits; and a p-channel transistor M7 for switching in the output stage.
The two p-channel transistors M1 and M2 constituting the current mirror section have their sources connected to a power supply VDD and their drains connected to the differential input section. The two n-channel transistors M3 and M4 constituting the differential input section have their gates connected to an output terminal OUT and an input terminal IN, respectively.
To the differential input section, connected is the n-channel transistor M9 which functions as a constant current circuit. This n-channel transistor M9 has its gate connected to a bias voltage source and its source connected to the ground GND. Also the n-channel transistor M10 which functions as another constant current circuit, has its gate connected to the bias voltage source and its source connected to the ground GND. The n-channel transistor M10 has its drain connected to the output terminal OUT together with the drain of the p-channel transistor M7 in the output stage.
The above described LDC source driver IC has a plurality of operational amplifiers arranged therein and each configured as shown in FIG. 1. The operational amplifiers, however, have offsets caused by manufacturing variation or the like, and the amount of offset varies from operational amplifier to operational amplifier. For this reason, neighboring operational amplifiers, which must originally output the same voltage value, may output different ones due to an offset in each operational amplifier. If this output deviation increases, irregularity in color on the display of the LCD may occur. The offsets in the individual operational amplifiers must be suppressed to prevent such irregularity in color.
To suppress offset in such an operational amplifier, the gate areas of transistors constituting the operational amplifier, in particular, the gate areas of the p-channel transistors M1 and M2 of the current mirror section and of the n-channel transistors M3 and M4 of the differential input section are conventionally increased to reduce the manufacturing variation relatively, thereby restraining the offset in the operational amplifier.
However, the amount of offset (xcex94Vgs) in the operational amplifier and the gate areas (S) of the transistors have the relationship xcex94Vgsxe2x88x9d1/{square root over ( )}S. For example, reducing the offset amount to half requires the gate areas of the transistors to be quadrupled. Thus, when such a conventional method is used to attempt to increase the definition of a recent LCD (a voltage range of several mV per gradation), the gate areas of the operational amplifier become very large, thereby increasing the chip area and cost.
It is an object of the present invention to suppress offset in an operational amplifier without increasing its gate areas.
An offset cancel circuit for an operational amplifier according to the present invention is characterized by comprising a capacitive element for storing a voltage to be amplified by an operational amplifier section and containing an offset and for feedback-controlling a voltage value of the operational amplifier section based on the stored voltage, and a switching element for switching operation between the storage operation for the voltage into the capacitive element and the feedback control operation based on the stored voltage value.
Since the present invention comprises the above technical means, when a voltage is input to an input terminal, each switching element is appropriately switched to store a voltage containing an offset, in the capacitive element. Each switching element is subsequently appropriately switched to provide such a feedback control that the gate voltages of the operational amplifier section become equal based on the voltage stored in the capacitive element, thereby canceling the offset in the operational amplifier section. Consequently, the offset in the operational amplifier section can be accurately canceled without increasing the gate areas of transistors in the operational amplifier section, thereby preventing chip area and cost from increasing. The present invention can therefore be applied to a source driver for a liquid crystal panel to prevent irregularity in color, thereby allowing the liquid crystal panel to be driven with a higher definition.